#2-56 UNC Torque Spec
Calculated preload torque by grade, plus what actually changes the number.
#2-56 UNC · Grade 5 · dry threads
0.338 lb-ft
0.458 N·m · targets 235
lb clamp load (75% of proof)
By grade
| Grade | Dry (lb-ft) | Lubed (lb-ft) | Dry (N·m) | Clamp load | Ultimate tensile |
|---|---|---|---|---|---|
| Grade 2 Low/medium carbon steel, unmarked head | 0.218 | 0.164 | 0.296 | 152 lb | 273 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 0.338 | 0.253 | 0.458 | 235 lb | 443 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 0.477 | 0.358 | 0.646 | 332 lb | 554 lb |
| 18-8 SS 304/18-8 stainless | 0.159 | 0.119 | 0.215 | 110 lb | 277 lb |
Why lubrication changes everything
Roughly 90% of the torque you apply is consumed by friction — under the head and in the threads. Only about 10% becomes useful bolt stretch. That means the friction coefficient, not the bolt, sets the relationship between torque and clamp load. Here is the same #2-56 UNC Grade 5 fastener under different conditions:
| Thread condition | Nut factor K | Torque for same preload |
|---|---|---|
| Plain / as-received, dry | 0.20 | 0.338 lb-ft |
| Zinc plated, dry | 0.22 | 0.371 lb-ft |
| Black oxide, lightly oiled | 0.18 | 0.304 lb-ft |
| Machine oil on threads | 0.15 | 0.253 lb-ft |
| Molybdenum disulfide / anti-seize | 0.12 | 0.203 lb-ft |
| Waxed or PTFE coated | 0.10 | 0.169 lb-ft |
Torquing a lubricated bolt to the dry spec overloads it — the
0.169
lb-ft row and the
0.338
lb-ft row produce the same clamp load. Apply the dry
number to a waxed fastener and you are at roughly twice the intended preload, which
for a Grade 5 is past yield.
Assumptions behind these numbers
T = K × D × F with K = 0.20, F = 0.75 × proof load, and proof load = tensile stress area (0.0037 in²) × the grade's proof strength. This is a general-engineering starting point for a reusable steel-on-steel joint. It is not a substitute for a manufacturer's torque sequence on an engine, a suspension component, a pressure vessel or any structural connection.